1. Field of the Invention
The present invention relates to a nonlinear optical device essential to an optical information processing system for controlling light by using optical or electrical signals and, more particularly, to a semiconductor quantum well optical device using a strained quantum well or superlattice structure of a compound semiconductor.
2. Description of the Related Art
To realize high-efficiency, high-speed nonlinear optical devices, a semiconductor quantum well optical device making use of a semiconductor quantum well or a superlattice structure has been researched and developed in recent years. The semiconductor quantum well or the superlattice structure has a large nonlinear constant and good electrical properties as compared with other nonlinear materials (e.g., organic nonlinear materials). Therefore, this structure enables to further enhance the optical nonlinearity by introducing some electrical means or to use an electrical device and a nonlinear optical device at the same time.
Recently, a strained quantum well or a superlattice structure has been proposed, which is formed along a [111] direction by using two types of compound semiconductors having different lattice constants, unlike conventional quantum well nonlinear devices formed along a [001] direction. In this structure, a high internal electric field produced by a piezoelectric effect caused by a lattice mismatch is present (for example, when the lattice mismatch is 1%, the strength of this internal electric field is about 10.sup.5 v/cm). When an external electric field is applied to this quantum well structure, therefore, a specific quantum confined stark effect (QCSE), e.g., a blue shift of an absorption peak due to the applied electric field is observed upon incident of light.
FIG. 1 shows a strained quantum well structure in which GaAs barrier layers 2 with no strain and an In.sub.1-x Ga.sub.x As well layer 3 with a compression strain are alternately formed on the (111) surface of a GaAs substrate 1 (K. W. Goosen, et al., Appl. Phys. Lett. 56(8), 715 (1990)). In this strained quantum well structure, the magnitude of a strain confined in the well layer 3, i.e., the magnitude of an internal electric field produced by the strain and the energy band offset (the depth of a quantum well) between the well layer 3 and the barrier layers 2 are determined by the composition of In contained in the well layer 3.
In conventionally proposed strained quantum well structures in a [111] direction as shown in FIG. 1, however, the strain produced by a lattice mismatch and the internal electric field produced by a piezoelectric effect caused by a lattice deformation are localized only in the well layer 3. For this reason, a large internal electric field exists even for a slight lattice mismatch. Since the magnitude of the internal electric field and the band offset are determined completely by the composition of In in the well layer 3, it is impossible to change the internal electric field and the band offset independently of each other.
In addition, according to the deformation potential theory, a direct transition type semiconductor increases its energy band gap when it is subjected to a compression strain. This consequently decreases the magnitude (band offset) of discontinuity at the ends of bands in the well layer 3 and the barrier layers 2, i.e., the depth of a quantum well. In this case, if the band offset, i.e., the composition of In is increased in order to keep the quantum confined effect, the strain increases accordingly, and the internal electric field also tends to increase to be larger than is necessary. That is, since both the internal electric field and the band offset strongly depend on the magnitude of a strain, it is impossible to change these factors independently of each other.
For the reasons described above, if the quantum well structure is applied to, for example, a SEED (Self Electric Optic Effect Device), which is the most representative nonlinear optical device, and no external electric field is applied to the quantum well, light absorption by excitonic transition becomes much weaker than that of a quantum well structure with no internal electric field. Therefore, modulation of light absorption derived from changes in (the direction and the magnitude of) the external electric field or optical bistability becomes smaller than those of regular quantum well devices.